An important criterion for measurements of Radar and Laser Radar (LADAR) is that the test target be in the "far field" of the transmitter, and the receiver be in the "far field" of the target. In the "far field" all paths of microwave or light waves are parallel. The required range can be deduced to be EQU R = D.sup.2 /L
where R is the range to the far field, D is the transmitter antenna size, and L is the wavelength of the radiation. With microwave radar, the state-of-the-art has been to provide a physical spacing sufficient to meet this condition. However, the very short wavelength (L) associated with LADAR means that the far field is at a long range, e.g., 100km for the case of a one meter aperture at 10.6 micron wavelength. This means that controlled measurements of test targets can be as difficult as operating an actual system.
Various proposals to make laser signature measurements have included either ignoring the errors due to the violating far field requirement, operating only with sub-scale targets, or such expensive approaches as flying the targets in balloons at 100,000 feet. It suffices to say that the state-of-the-art prior to this invention offered no satisfactory solution to the problem.
Therefore, it is an object of this invention to make possible the measurement of the far field properties of full scale LADAR targets.
Another object of this invention is to make it possible through appropriate use of optics to make laser signature measurements with Fourier Transforms.
Still another object of this invention is to generate far field information at ranges of approximately 10 meters.
Still another object of this invention is to provide an optical reflective device that can be placed inside in a controlled environment so that more accurate data can be obtained in all weather conditions.
Still another object of this invention is to provide an optical reflective device which needs no airborne or balloon instrumentation and there is no requirement for the development of pointing and tracking systems just to make static measurements.